Monolithic crystal filters are well known in the radio communications art. FIG. 1 shows a top view of a prior art two-pole monolithic crystal filter (10).
The filter (10) shown in FIG. 1 is comprised of a single, piezoelectric substrate (12) which is typically quartz material. The upper and lower surfaces of the substrate (12) are usually planar and has on its upper surface planar electrodes (14 and 16) which comprise input and output nodes (2 and 3 respectively) of the filter (10).
These electrodes (14 and 16) form resonators with the addition of ground or reference electrodes (15 and 17) that are deposited on the opposite side of the substrate (12), directly below the upper surface electrodes (14 and 16). The reference electrodes are shown in FIG. 1 in the broken, and dashed lines. The reference potential electrodes are connected to a common reference potential node (19) which is also shown in broken lines indicating that it is on the opposite side of the substrate (12).
FIG. 2 shows the electrical equivalent circuit 20 for the two-pole monolithic filter (10) shown in FIG. 1. The input node in FIG. 2 is identified by reference numeral (2) and corresponds to the input node identified by the same reference numeral in FIG. 1. The output node shown in FIG. 2 is identified by reference numeral (3) and corresponds to the output node identified by the same reference numeral in FIG. 1. The reference potential node is identified in FIG. 1 and FIG. 2 by reference numeral 19.
The input resonator shown in FIG. 1 (comprised of electrodes 16 and 17) has an electrical equivalent shown in FIG. 2 that is comprised of a shunt capacitance (21) and a series inductance (22), a series capacitance (24) and a series resistance (26). The output resonator shown in FIG. 1, (comprised of electrodes 14 and 15) has an electrical equivalent shown in FIG. 2 as series resistance (22'), series capacitance (24') and series inductance (26') and shunt capacitance (21') identify the like elements of the other resonator stage. The shunt inductance (13) represents the acoustic coupling between the two resonator stages that is accomplished by means of the piezoelectric effect coupling the two resonator stages together through the substrate (12) shown in FIG. 1.
A problem with a two-pole monolithic piezoelectric filter, such as the device shown in FIG. 1, is that it may not provide enough signal attenuation for a particular radio frequency communications device. To increase the attenuation of intermediate frequency (IF) undesirable signals it has heretofore been the practice to use either a four-pole device, or to cascade two or more two-pole devices to provide a sufficiently steep frequency response for a radio communications application. Using four-pole or multiple two-pole devices to achieve a desired frequency rejection characteristic increases the size and cost of the filter or filters.
In FIG. 3, there is shown a representative plot of the attenuation of a single two-pole monolithic piezoelectric filter in the plot identified by reference numeral (32). Such a single two-pole device, which may not sufficiently attenuate input signals for a particular application, for instance, in dual-conversion receivers and at twice the frequency of the second local oscillator, as shown in FIG. 3. The trace shown in broken lines and identified by reference numeral (34) has a much sharper attenuation but it is at the expense of an additional two-pole of filtering and an overkill in cost, radio size and radio weight, if the only additional attenuation needed is at frequency of F.sub.c +2F.sub.lo where F.sub.lo is the second local oscillator frequency.
A piezoelectric filter device that provides an improved attenuation of input signal but that requires less volume than either multiple individual two-pole or four-pole piezoelectric filters would be an improvement over the prior art.